It's a great idea but not fully baked yet. My former colleague is using it for internal apps and it seems to work fairly well (UI is not fancy), but not for anything going into the high volume apps, which has much more complex behavior (designers are asking for stupidly complex UI). It's much better in the upcoming OS since they added better navigation, but requires the latest OS, which is not supportive for many companies who try to support back at least 1 year.
Will it ever be mainstream for iOS/MacOS? Maybe, if Apple would use it way more than they have so far, thus improvement by dogfooding. It is nice to build apps this way, but there are way too many bizarre gotchas still for most people unless you can live with more flexible designs.